1. Field of the Invention
The present invention is generally related to processes and apparatus used in photochemical vapor deposition (photo-CVD) of materials on substrates, and more particularly to removing and preventing deposition on a transparent solid medium, such as a window, that is positioned between the photon source and the photolysis region, which film, if not prevented or removed, inhibits transmission of photon energy to the photolysis region during deposition processes.
2. Description of the Prior Art
It has been found recently that some materials that are already used, or having potential for use, in thin film semiconductor devices can be deposited on substrates with a photo-CVD technique. In processes using this technique, a substrate is placed in a vacuum chamber, and a reaction gas containing atoms or molecules of the material to be deposited on the substrate is injected into the vacuum chamber. The reaction gas is then exposed to light energy, such as ultraviolet (UV) light, visible light, or infrared radiation, which breaks the molecular bonds and leaves the desired atomic or molecular species to be deposited free to bond with the substrate or with other atoms or molecules of the deposited material already on the substrate. For example, solar cells composed of a thin film of hydrogenated amorphous silicon (a-Si:H) on a substrate can be fabricated by exposing disilane gas (Si.sub.2 H.sub.6) to UV light in a vacuum chamber containing the substrate. The photon energy from the UV light breaks Si-H or Si-Si molecular bonds in the Si.sub.2 H.sub.6 gas, thereby freeing Si atoms to bond with other silicon atoms deposited on the substrate to build up a film of a-Si:H.
Such a photo-CVD process for producing a-Si:H films has been shown recently to produce film properties and solar cell efficiencies similar to the best a-Si:H films produced by glow discharge processes. See M. Konagai, MRS SYMP. PROC. 70, 257 (1986). Further, since this process does not involve high voltage ion bombardment, as required by glow discharge deposition, there is no ion bombardment of the substrate surface, chamber walls, and RF electrodes that causes film structural damage and impurity contamination to the deposited film. Therefore, there are substantial reasons for developing the photo-CVD process for commercial production of thin films.
However, prior to this invention, there was still a significant problem associated with the photo-CVD process that precluded efficient commercial use. While depositing the film of the desired atomic or molecular species on the substrate, a film of that material also deposits on glass or other transparent materials through which the UV or other light is introduced into the vacuum chamber. For example, if UV source light bulbs or tubes are positioned in the chamber where photo-CVD of a-Si:H is being performed, a film of a-Si:H also builds up on the surfaces of the light bulbs or tubes. On the other hand, where a transparent window is provided in the side of the vacuum chamber, and the UV light source is positioned outside the window, an a-Si:H film builds up on the inside surface of the window. In either case, the thicker the film build-up, the more it inhibits transmission of the UV light to the Si.sub.2 H.sub.6 process gas, thus decreasing the photolysis and the efficiency of photo-CVD process and eventually effectively shutting down the process.
As a result, in order to continue the photo-CVD process, the vacuum chamber has to be opened to wipe or clean the deposited film from the window or bulbs, sometimes before the desired film on the substrate is even completed, particularly if a somewhat thicker film is desired. Such shut-down and opening of the vacuum chamber to clean the film off the window or bulbs is not only inefficient and labor intensive, but it is also detrimental to the integrity of the film being produced on the substrate. Specifically, impurities, such as oxygen, water vapor, aerosols, and other substances in the air degrade the desired film on the substrate. Even when one thin film can be completed before the deposition on the window or bulb totally blocks the light source, the chamber still has to be opened and cleaned before the next thin film on a substrate can be produced. Once the chamber is opened, it requires closure, pump down, and overnight heating to eliminate the impurities before another film can be produced. Therefore, it has still been a very inefficient, labor intensive process that is not conducive to commercial production. In order for photo-CVD to be a viable manufacturing technique, film deposition on the transparent window through which light is introduced to the vacuum chamber has to be eliminated.
There have been a number of attempts to solve this problem prior to this invention. All of the attempts have been effective to some extent, but also have created new problems or have not completely solved the existing problem. For example, a number of attempts have been made to solve the problem by blowing an inert purge gas, such as helium (He) on the interior surface of the transparent window in an attempt to keep the process gas away from the window. See, e.g., A. Yoshikwaw, et al., 23 JPN. J. APPL. PHYS. L91 (1984), H. Zarnani, et al., 60 J. APP. PHYS. 2523(1986), J. M. Jasinski, et al., 61 J. APPL. PHYS. 431 (1987), K. Kumata, et al., 48 APPL. PHYS. LETT. 1380 (1986), K. Tamagawa, et al., 25 JPN. J. APPL. PHYS. L728 (1986), and Y. Numasawa, et al., 15 J. ELECT. MAT. 27 (1986). The advantages of such an inert gas purge next to the window are that it does not introduce degrading impurities to the film being produced on the substrate and that it does not require moving parts. A significant disadvantage of this inert gas purge is that film deposition on the transparent window is only retarded and not prevented completely. Therefore, it does not always keep the film off the window long enough to complete a normal deposition process, especially where a thicker film on the substrate is desired, and the chamber still has to be opened, cleaned, reevacuated, and heated overnight between each substratecoating process. Also, in order to retard the film growth on the window enough to be beneficial, this inert purge technique requires large purge gas flows. Such large purge gas flows dilute the process gas, which is usually quite expensive, thus reducing efficiency of material usage. Such substantial dilution of the process gas can also adversely affect the film growth process on the substrate. This purge technique is better suited to laser photolysis because the beam can be focused to a small area at the window, as reported by A. Yoshikawa, et al., supra, H. Zarnani, et al., supra, and J. M. Jasinski, et al., supra. T. Saitoh, et al., 42 APPL. PHYS. LETT. 678 (1983), reported that a somewhat thicker film deposition on the substrate can be obtained by repetitively plasma etching the window and resuming the deposition. However, such plasma etching in the chamber requiring periodic interruption of the photo-CVD process is inefficient, can produce undesirable impurities, and detracts from the benefits of photo-CVD over normal plasma deposition.
Another approach to solve the problem, as reported by T. Inoue, et al., 43 APPL. PHYS. LETT. 744 (1983), and A. E. Delahoy, 77 & 78 J. NON-CRYST. SOLIDS 322 (1985), has been to coat the interior surface of the window with a transparent film of low vapor pressure oil, such as Fomblin, to reduce the sticking coefficient of the material being deposited. This oil coating technique has better success at retarding film growth on the window than the inner gas purge technique, but carbon from the oil is a source of degrading impurity that can have a deleterious effect on the film being grown on the substrate. Also, while the oil coating does retard film growth on the window, it still provides only enough time to deposit about a 3-.mu.m film on the substrate. Thus, one successful substrate coating is still about all that can be expected before the chamber has to be opened again for cleaning. The U.S. Pat. No. 4,597,986, issued to R. Scapple, et al., describes an improvement whereby oil is continually applied to the window surface while the surface is wiped with a wiper blade. This latter improvement does enhance continuous production, but the carbon impurity problem renders this technique unsuitable for deposition of semiconductor films that require a high degree of purity.
U.S. Pat. No. 4,265,932, issued to Peters, discloses still another approach in which a movable UV transparent sheet is positioned between the process gas and the window so that a film is deposited on the movable sheet instead of on the window. The clean sheet is continuously unwound from a spool and drawn across the window, then wound onto another spool during the photo-CVD process so that no film build-up to inhibit UV light entering the chamber is allowed. This technique is quite effective for one substrate. Its only disadvantages are that it takes about 300 feet of sheet for the time it takes to accomplish one film deposition on a substrate, so the chamber still has to be opened after every run to change the roll of transparent sheet, and there is still the possibility of some contaminants that emanate or outgas from the sheet as it is unrolled. The U.S. Pat. No. 4,654,226, issued to R. Rochelea, et al., discloses an improvement on this movable sheet technique.
Another interesting approach illustrated by the U.S. Pat. No. 4,454,835, issued to P. Walsh, et al., is to avoid the problem by incorporating a UV light source right in the vacuum chamber without any intervening transparent windows, sheets, or bulbs. In this kind of apparatus, a lamp gas, such as argon or neon, is introduced directly into the vacuum chamber near some discharge electrodes while the process gas is introduced into the chamber near the substrate. The lamp gas is ionized right in the vacuum chamber to create a glow discharge along side the process gas by the electrodes to emit the required UV light, while the film is deposited from the process gas onto the substrate. Since the lamp and deposition regions are in a common vacuum chamber, it is difficult to distinguish between the effects of a remote plasma that may include at least some of the process gas and photolysis of the process gas. This type of system may yet become successful in producing high quality films, but the deposition mechanism is still uncertain, and since the lamp and process gases do mix in the vacuum chamber, the problem of impurities still has to be considered.
Consequently, while the actual photo-CVD process has great potential for producing at least some very commercially desirable thin film semiconductor devices, such as the a-Si:H solar cells discussed above, there has still been a critical need for an effective method and apparatus for eliminating the film build-up on the vacuum chamber window where UV light is introduced in order to make this photo-CVD process commercially viable. Such a solution should meet at least four criteria, as follows: (1) It should not reduce window transparency; (2) It should eliminate deposition on the window while not adversely affecting deposition on the substrate; (3) It should be inert or benign to the film deposition on the substrate and not introduce undesirable impurities into the chamber which would degrade the film deposited on the substrate; and (4) It should be continuously effective without requiring periodic opening of the chamber so that efficient, continuous production of film depositions on successive substrates can be accomplished.